According to the police report the officer made, Mr. Gates never touched him and officer was never in physical danger. Nor was he inciting a riot or other neighborhood uprising.

Did he arrest him in order to save face? If he objected to Mr. Gates' "loud and tumultuous voice", all he had to do was leave.

"It is not against the law to mouth off at a policeman.   It's not smart, but it's not against the law. So the job of the police is not easy." Ron Davies

"The law is aimed not at mere irascibility but rather at unruly behavior likely to set off wider unrest. Accordingly, the behavior must take place in public or on private property where people tend to gather. While the police allege that a crowd had formed outside Gates' property, it is rare to see a disorderly conduct conviction for behavior on the suspect's own front porch." heric

Being a jerk - even a loud one - is not illegal. I suspect that a white person has no concept of what it is like to be confronted by a White police officer while being Black.